Terephthalic acid and other aromatic carboxylic acids are widely used in manufacture of polyesters, commonly by reaction with ethylene glycol, higher alkylene glycols or combinations thereof, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid-phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product, with air or another source of oxygen, which is normally gaseous, in the presence of a bromine-promoted catalyst comprising cobalt and manganese. The oxidation is exothermic and yields aromatic carboxylic acid together with byproducts, including partial or intermediate oxidation products of the aromatic feedstock, and acetic acid reaction products, such as methanol, methyl acetate, and methyl bromide. Water is also generated as a byproduct. Aromatic carboxylic acids, typically accompanied by oxidation byproducts of the feedstock, are commonly formed dissolved or as suspended solids in the liquid-phase reaction mixture and are commonly recovered by crystallization and solid-liquid separation techniques.
The exothermic oxidation reaction is commonly conducted in a suitable reaction vessel at elevated temperature and pressure. A liquid-phase reaction mixture is maintained in the vessel and a vapor phase formed as a result of the exothermic oxidation is evaporated from the liquid phase and removed from the reactor to control reaction temperature. The vapor phase comprises water vapor, vaporized acetic acid reaction solvent and small amounts of byproducts of the oxidation, including both solvent and feedstock byproducts. It usually also contains oxygen gas not consumed in oxidation, minor amounts of unreacted feedstock, carbon oxides and, when the oxygen source for the process is air or another oxygen-containing gaseous mixture, nitrogen and other inert gaseous components of the source gas.
The high temperature and pressure vapor phase generated by liquid-phase oxidation is a potentially valuable source of recoverable acetic acid reaction solvent, unreacted feed material and reaction byproducts, as well as energy. However, its substantial water content, high temperature and pressure and corrosive nature due to components such as gaseous methyl bromide, acetic acid solvent and water pose technical and economic challenges to separating or recovering components for recycle and recovering its energy content. Further, impurities that remain unseparated in recovered process streams can prevent re-use of streams if impurities adversely affect other process aspects or product quality.
Purified forms of aromatic carboxylic acids are usually favored for the manufacture of polyesters for important applications, such as fibers and bottles, because impurities, such as the byproducts generated from the aromatic feedstocks during oxidation and, more generally, various carbonyl-substituted aromatic species, are known to cause or correlate with color formation in polyesters made from the acids and, in turn, off-color in polyester converted products.
Preferred purified forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents, such as purified terephthalic acid or “PTA”, are made by catalytically hydrogenating less pure forms of the acids, such as crude product comprising aromatic carboxylic acid and byproducts generated by the liquid-phase oxidation of the aromatic feedstock or so-called medium purity products, in solution at elevated temperature and pressure using a noble metal catalyst. Purification not only removes impurities from the crude and medium purity products, particularly the major impurity, 4-carboxybenzaldehyde, but also reduces the level of color bodies and the amount of metals, acetic acid and bromine in commercial practice, liquid-phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which crude product from liquid-phase oxidation is used as the starting material for purification.
Over time, advances have been made to processes for the production of high purity aromatic carboxylic acids. For instance, benzoic acid has been found to be a viable alternative oxidation solvent since it is relatively resistant to oxidation. U.S. Pat. No. 6,562,997 discloses a process for the production of aromatic dicarboxylic or tricarboxylic acid in which the formation of methyl bromide is substantially reduced relative to conventional processes through the use of benzoic acid as part of the solvent. This reference also teaches that water and benzoic acid are more easily separated due to the differences in their boiling points versus, e.g., water and acetic acid, thereby substantially reducing the complexity of fractionation of the acid and water.
Another notable advance has been the elimination of process steps between oxidation and purification. U.S. Pat. No. 4,675,438 teaches the direct purification of iso- or terephthalic acid products from the oxidation of m- or p-xylene with air in the presence of a combination of bromine with cobalt and manganese and a solvent comprising 85 to 97 percent benzoic acid and 15 to 3 percent water. The direct purification involves diluting the fluid oxidation effluent with water to provide a solvent system of from 25 to 75 weight percent water and from 75 to 25 weight percent benzoic acid, heating the diluted fluid effluent to a temperature at which all of the solids in the oxidation effluent dissolve in the solvent system, and hydrogenating the solution in the presence of a Group VIII noble metal catalyst. Unfortunately, although the low water solvent in the oxidation step facilitates oxidation since water tends to deactivate the catalyst, it does not keep the iso- or terephthalic acid in solution. Therefore, in order to process the oxidation effluent through the hydrogenation step, additional process steps and equipment are still required to add water and heat the effluent to dissolve the iso- or terephthalic acid. This water addition causes the mother liquor from the hydrogenation step to be very high in water content, thereby requiring its subsequent removal in order to maintain the low water solvent in the oxidation step.
Accordingly, it would be desirable to provide a process for the production of at least one high purity aromatic carboxylic acid which not only eliminates the need for intermediate crystallization and solid-liquid separation techniques between the oxidation and purification steps, but also uses the same solvent composition for both oxidation and purification and keeps the aromatic and carboxylic acid in solution through both of these steps. By utilizing the same solvent for both steps, the need to (1) add both water and heat to the oxidation effluent, and thus additional process steps and equipment, in order to dissolve the aromatic carboxylic acid prior to purification and (2) remove water after purification would be eliminated.